Multi-select on markdown notebook cells breaks selection

[miguelsolorio]

The new markdown cell renderer seems to break multi-select. Here I start selecting via holding the Shift key and only when I select a markdown cell, my selection is cleared. This goes away when i disable the new markdown renderer.

[mjbvz]

We likely need to update this code:

vscode/src/vs/workbench/contrib/notebook/browser/view/renderers/backLayerWebView.ts

Line 891 in 2cde743

const cell = this.notebookEditor.getCellById(data.cellId);

Currently it always focuses the cell, but if shift or mata is held down, we should instead add the cell to the selection. I made sure the event data is passed back to the renderer, but wasn't sure how to actually update the selection in a notebook programatically. Let's take a look for April